Adolescent pregnancy is a significant problem in the United States. Special nutritional concerns are present in this group to insure that optimal nutritional needs can be met by both the fetus and the growing adolescent. The additional nutritional demands of pregnancy may be of primary concern when the mother has not yet completed her linear growth and has not full achieved her peak bone mass. The aim of the proposed study is to address the impact of pregnancy and lactation on the efficiency of calcium absorption and rates of bone calcium deposition and resorption in adolescent non-Hispanic white and non-Hispanic black girls between the ages of 11-17 years. Fifteen girls from each group who intend to breastfeed their infants will participate in a longitudinal calcium kinetic study designed to address the efficiency of calcium absorption and rates of bone calcium turnover during both the third trimester of pregnancy (34-36 weeks of gestation) and during lactation (1-2 months post partum). All adolescents will be screened for positive predictors of lactation performance prior to the start of the study. In each kinetic study, girls will be admitted to the General Clinical Research Unit at Johns Hopkins Hospital. On the morning of each study, girls will consume a glass of milk containing a stable calcium isotope (46Ca), and following breakfast a second stable calcium isotope (42Ca) will be administered intravenously. Timed blood samples will be obtained for eight hours following the isotope dosing, and a complete 24 hour urine collection will be made. Additional spot urine samples will be collected for 5 days following each isotope study. To address the hormonal response to pregnancy and lactation, calcitropic hormones will be measured during each study using a fasting blood sample. Rates of bone calcium deposition and resorption in response to these physiological changes will be determined using multicompartmental modeling. These studies will provide novel information on the physiological alterations in calcium absorption and bone calcium turnover which occur to support both the increased calcium demands of adolescence and the calcium demands of pregnancy and lactation.